The present invention relates generally to disc drives and more specifically to hard disc drives (HDD) that include rotatable actuators.
The rotary actuator in a HDD is typically supported by a bearing pivot mechanism consisting of two preloaded ball bearings. The actuator is used to position magnetic transducers (heads) over selected information bearing tracks on the discs. The transducers have to be positioned with great precision and the actuator positioning is controlled by a closed loop servo system with its movement being driven by a voice coil motor (VCM). The feedback in the control loop is through the transducer reading servo information pre-written on the disc. During track following or track to track seek operations, the pivot bearing may rotate less than one minute to as much as 20xc2x0 in an inner to outer radius seek or reverse.
The friction torque generated in the bearings adversely affects the servo control system, especially in high density track applications. In modeling of the HDD actuator, the actuator is often simplified as a double integrator P(s)=K/s2. However, the non linear response of the actuator""s ball bearing affects the form of the transfer function. In particular, there is insufficient gain at the lows frequency and the gain at the low frequency reduces with decreasing the actuator motion. As a result of this, the servo system is unable to handle high track density applications with ball bearing pivot mechanisms.
Various alternatives such as knife edge type pivots have been proposed to meet this problem. One example is disclosed in U.S. Pat. No. 5,355,268 entitled xe2x80x9cDisc drive knife edge pivotxe2x80x9d by D M Shultz, granted Oct. 11 1994. Other alternatives such as micro actuators, or modified actuator arm designs such as that disclosed in U.S. Pat. No. 5,166,850 entitled xe2x80x9cRigid, wedge-shaped mounting structure for minimizing resonances to allow rapid transverse movement of an attached headxe2x80x9d have also been proposed.
A problem with such proposals is that major changes are required to existing designs, which increase manufacturing costs and also time to market.
The present invention provides a disc drive which includes a system to minimize the effect of bearing friction and which may be easily incorporated into existing disc drive designs.
In accordance with one embodiment of the invention there is provided a disc drive which includes a rotatable disc operative to include a plurality of data storage tracks. A rotatable actuator assembly is mounted via a pivot mechanism to a base of the hard disc drive and this assembly includes an actuator body and a transducer mounted on the actuator body. A voice coil motor is operatively connected to the actuator arm and operable to position the transducer over a selected storage track during a track following operation where the transducer is able to retrieve data from, or store data on, the selected track. The radial distance between the center line of the selected data storage track and the spindle axis varies on angular displacement about the spindle axis so that the actuator arm is operable to move about the pivot mechanism during the track following operation.
In previous disc drive systems, the data storage tracks are circular and concentric about the axis of rotation of the disc thereby enabling the actuator arm to be stationery during the track following operation. The center line of the data storage tracks are typically defined by servo information which is prewritten into the disc of the disc drive, and which is used in operation of the disc drive by the controlling servo system. However, by maintaining the actuator arm in motion during track following operations, it is possible to increase the gain at the low frequency and also to minimize the gain variation with the amplitude change of the actuator motion. This enables the servo system associated with the disc drive to be able to be used on discs having higher track density applications than previously possible, using existing designs incorporating ball bearing pivot mechanisms.
In another embodiment of the invention, there is provided a method of retrieving data from, or storing data on, a data storage track of a disc drive. The method includes providing a rotatable actuator assembly which is mounted via a pivot mechanism to a base of the disc drive and which includes an actuator body and a transducer. The method also includes positioning the transducer over a data storage track during a track following operation, and causing the actuator assembly to pivot about the pivot mechanism during the track following operation where the transducer retrieves data from, or stores data on, the storage track.
In one form, the data storage tracks are defined to be non circular about the disc""s axis of rotation so as to cause the actuator arm to pivot during the track following operation. This may be achieved in different ways. In one arrangement the disc incorporates circular data storage tracks but the disc is designed to rotate about an axis which is offset from the central axis of the data storage tracks. In another form, the data storage tracks are defined to be non-circular about the axis of rotation of the disc. This is preferably controlled by the servo information which defines the center line of the data storage tracks which is written in during servo track writing of the hard disc drive.
Preferably, the position of the data storage track is defined by servo track information which is written into the disc drive. In one form, a time varying signal is injected in during this servo track writing. Preferably this signal is a periodic time varying signal and the spindle rotation frequency or its harmonic frequencies are selected as the frequency of the written in periodic signal. In a preferred form the signal is sinusoidal.
Preferably, at the time of writing in the signal into the servo track, a correction factor for the servo system is created and stored and utilized by the servo system during track following operation. Preferably, the same amplitude and phase synchronized signal is written in for all tracks to thereby ensure that no additional track squeeze is generated.
Preferably the amplitude of movement of the actuator arm during track following operations is in the range of (100-300)xcexcxe2x80x3 of motion at the transducer which for a typical disc drive amounts to [(2.73-8.19)xc3x9710xe2x88x923]xc2x0 of arm rotation.
In yet a further embodiment, the present invention relates to a method of creating a servo track on a rotating disc of a disc drive. The method includes the steps of providing an actuator assembly having a transducer for writing in the servo track on the rotatable disc. A voice coil motor is operatively connected to the actuator assembly and operable to move the transducer in response to an input signal. The method further includes injecting a time variable input signal to the voice coil motor to cause the transducer to move during writing of the servo track so that the radial distance between a center line of the servo track relative to the axis of rotation of the disc varies on angular displacement about the axis.
Preferably the method further includes creating a correction factor for the servo system which is utilized by the servo system during track following operation.